all of the suggested answers are very good. here are a few more
don't take it personally
don't get your back up
don't get bent out of shape
don't get your nose out of joint
don't get your panties in a wad
(yes, used also for guys!)
Geez, Louise, relax
take a chill pill
Depends on your context, and how contemporary it needs to be.
For a good basic expression, I think I like Sonia's "taking it in good humor the best."
Ref: personal experience :-)
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